1. Field of Invention
The invention concerns a method to control the emission of an electron beam in an X-ray imaging tube. The invention also concerns an electron-emitting cathode, a tube, and an X-ray imaging system to implement said method.
2. Description of Related Art
In known X-ray imaging systems, such as scanners for medical applications, an imaging tube emits X-rays which pass through an object to be observed, e.g. part of a patient's body, said X-rays then being detected by an array of detectors which allows an image of the object to be constructed.
Imaging tubes generally consist of a cathode capable of emitting an incident beam of electrons onto an impact focal spot on an anode, the cathode and the anode being separated by a vacuum.
The cathode typically comprises an electron-emitting device, consisting for example of a coiled tungsten filament and heated to high temperature via an electric current, which allows the beam of electrons to be generated. The electrons are then accelerated in the vacuum between the cathode and anode by means of an electric voltage difference applied between the cathode and anode.
The anode, typically a target in tungsten, which may rotate, then generates the X-rays after its interaction with the beam of electrons.
The design of the cathode is subject to various contradictory constraints, depending upon the use made of the associated imaging system. The constraints differ if the imaging system is used in neurology, cardiology or mammography for example.
One constraint is that the emission device of the cathode must generate a beam of electrons containing a sufficient number of electrons in order to obtain good image quality. The number of electrons in the beam depends, in particular, on the temperature of the filament of the emission device, called the emission temperature. To generate a beam containing a higher number of electrons, a large filament is most often used since it can emit a higher number of electrons at one same temperature and hence at one same rate of consumption or evaporation. Evidently, the increase in temperature is made to the detriment of the lifetime of the emission device, whose filament may finally break after evaporation at high temperature.
A further constraint is the size of the focal spot of the electron beam on the anode. The smaller the focal spot, the greater the resolution of the final image since finer details will be able to be distinguished.
Reducing the size of the impact focal spot can be achieved passively i.e. using a smaller filament and/or arranging the filament of the emission device in a focusing cup whose mechanical profile, e.g. stepped shape, allows the beam to be concentrated, and/or it can be achieved actively which is generally obtained by applying a negative voltage to polarizing plates located in the vicinity of the emission device which allows the beam of electrons to be concentrated onto a smaller focal spot by means of electrostatic forces.
Some applications, in particular medical observation applications of a patient via the X-ray imaging system require a beam of electrons emitted in different emission modes which persons skilled in the art call “fluoro” emission mode and “record” emission mode.
The fluoro emission mode is a pulsed emission mode of the beam of electrons, used when observing a patient over a long period or over a large part of the body, for which the patient's radiation dose must be reduced and for which low image quality is sufficient. Therefore, in the fluoro mode one possibility consists of reducing the radiation dose in each pulse, so as to minimize the patient's radiation dose during exposure. This is also called low power pulsed emission mode. It is also possible to reduce non-necessary patient radiation by controlling the duration of each pulse through the periodic application of a cut-off voltage to a focusing cup located in the vicinity of the emission device, which allows blocking of the electrons in the emission device through the repelling effect of electrostatic forces. Therefore, between two pulses, the emission of the electron beam is cut off and there is no patient radiation. This also takes part in better image quality.
The record emission mode is a pulsed emission mode of a beam of electrons, used when observing a patient over a short period of time or over a small part of the body, for which good image quality is necessary. Good image quality requires high irradiating power. In this case the term high power pulsed emission mode is used. Between the taking of two images, the high acceleration voltage of the electron beam is cut off. However, no cut-off voltage is applied to the focusing cup, which may generate remanent radiation for the patient since the cut-off of the high acceleration voltage is not instantaneous.
One disadvantage of prior art solutions is that they do not offer emission devices allowing simultaneous active reducing of focal spot size and a fluoro emission mode. In general, prior art tubes have a first emission device comprising a small filament located in a focusing cup to which a cut-off voltage can be applied for observation in fluoro mode, and a separate second emission device comprising a large filament for observation in record mode.
These solutions are therefore not flexible, and since the fluoro mode is used for long periods and is the most utilized mode in the imaging system, the small filament emission device is given intensive use, which reduces its lifetime.
Another drawback of some prior art solutions is that they do not allow the reconciling of high emission temperature to obtain good image quality (which generally assumes the use of a large filament) with the obtaining of a small-size focal spot, also to ensure good image resolution, whilst maintaining a long lifetime of the emission device.